1. Field of the Invention
The present invention relates to a controller for controlling operation of a pulse width modulation signal-driven device and a method of reducing unnecessary noise spectrum component of the signal-driven device.
A device driven with a pulse width modulation signal (for example, a current controlled-stepping motor) is so structured that operation of the above device is controlled by varying duty ratio of waveform of a current flowing in the above device, namely, pulse width modulation (hereinafter referred to as “PWM”). Switching a drive current of a load at PWM pulse trains, the above PWM signal-driven device causes a switching noise of a reference control frequency and of a high harmonic (of the reference control frequency). The thus caused switching noise is inconvenient. In a vehicle, for example, the switching noise may influence hearing of an on-vehicle radio or on operation of other digital devices.
2. Description of the Related Art
Japanese Patent Unexamined Publication No. 1998-99795 (JP7099795) discloses a controlling device for stepping motor (referred to as “control device 5”), where a method of reducing noise spectrum component is shown. In JP7099795, for controlling a current controlled-stepping motor, a sinusoidal wave frequency modulation is applied to a PWM drive current pulse train (hereinafter, as the case may be, referred to as “control clock”), where the sinusoidal wave has a frequency lower than that of the control clock. With this, a noise spectrum component is diffused in a desired frequency band, thereby deceasing influence on the on-vehicle radio or other digital devices.
The above noise-reducing method according to the related art is, however, not necessarily sufficient for decreasing the influence on radio hearing, for the following reason. As a communication device, for example, the AM broadcasting in Japan uses frequency band of 545 kHz to 1,605 kHz, where multiples of 9 kHz are distributed to a broadcasting carrier wave of each office. In addition, about ±6 kHz of the broadcasting carrier wave is a side band wave, namely, about 12 kHz is a band width of one broadcasting carrier wave. Specifically, Nippon Broadcasting System, Inc. capable of receiving communications in and around Tokyo has a broadcasting carrier wave of 1,242 kHz, or 1,236 kHz to 1,248 kHz including the side band wave.
Herein, a controller of a stepping motor having a control clock frequency of 248 kHz is to be taken for example, so as to check for a possible influence on radio hearing by a noise of the control clock frequency and of the high harmonic (of the control clock frequency). In this case, the control clock has its five-multiple high harmonic frequency of 1,240 kHz (=248 kHz×5). This 1,240 kHz falls within the broadcasting carrier wave of Nippon Broadcasting System, Inc., (1,236 kHz to 1,248 kHz) including the side band wave, and therefore, this high harmonic component enters the radio's voice output as a noise, causing a harmful effect on the radio hearing of the broadcasting office. When a peak of a noise spectrum attributable to the control clock (and even if this control clock is out of the side band wave) is diffused by the method according to the related art, the thus diffused noise spectrum component is included in the broadcasting band, thus increasing influence on the radio hearing. As described the above, diffusing the spectrum of the switching noise of the control clock of the PWM signal-driven device in a frequency range may fail to sufficiently decrease influence on the radio receiving and on operation of other on-vehicle devices.